Systems are known for measuring those characteristic attitude angles of the wheels of an automobile frame which enable convergence to be determined and the thrust axis defined.
These known systems use devices comprising, inter alia, light beam emitting means in combination with angle transducer means of the optical type which are fixed to the wheel rims by suitable brackets and interact with the transducers of the adjacent wheels, both on the same side and on the opposing sides of the automobile frame, by feeding and receiving light beams parallel to the equatorial planes of the wheels and measuring the angles of incidence of said beams.
There is however one attitude quantity, depending on its geometrical characteristics, which is not possible to measure by known systems. This quantity is the wheel base, which because of the inclination of the steering axis, varies as the steering angle varies.
It should be noted that the term "steering axis" means the axis about which the overall wheel swivels when steered.
The steering axis is generally inclined both in the transverse plane perpendicular to the automobile frame axis and in the longitudinal plane parallel to said axis.
The inclination is such that the steering axis is generally incident with the surface on which the frame rests at a point external to the equatorial wheel plane, and in front of the point of contact of the diametrical plane with the resting surface in the direction of movement.
The so-called transverse and longitudinal wheel offsets are determined by projecting the distance of the point of incidence from the point of contact into the transverse and longitudinal planes passing through the center of the wheel equatorial plane (wheel center of rotation).
In accordance with well known parameters the values of the wheel offsets determine the directional stability of the automobile frame during both straight and curved movement. As these offsets vary in accordance with the wheel steering angle, as does the wheel base, the corresponding offset variations can be obtained by measuring the wheel base variations as a function of the steering angle. This indicates the importance of being able to measure the wheel base.